Image sensor with a high absorption layer

ABSTRACT

An image sensor with high quantum efficiency is provided. In some embodiments, a semiconductor substrate includes a non-porous semiconductor layer along a front side of the semiconductor substrate. A periodic structure is along a back side of the semiconductor substrate. A high absorption layer lines the periodic structure on the back side of the semiconductor substrate. The high absorption layer is a semiconductor material with an energy bandgap less than that of the non-porous semiconductor layer. A photodetector is in the semiconductor substrate and the high absorption layer. A method for manufacturing the image sensor is also provided.

REFERENCE TO RELATED APPLICATION

This Application is a Divisional of U.S. application Ser. No.15/609,325, filed on May 31, 2017, the contents of which are herebyincorporated by reference in their entirety.

BACKGROUND

Integrated circuits (ICs) with image sensors are used in a wide range ofmodern day electronic devices, such as, for example, cameras and cellphones. In recent years, complementary metal-oxide semiconductor (CMOS)image sensors have begun to see widespread use, largely replacingcharge-coupled device (CCD) image sensors. Compared to CCD imagesensors, CMOS image sensors are favored due to low power consumption,small size, fast data processing, a direct output of data, and lowmanufacturing cost. Some types of CMOS image sensors include front-sideilluminated (FSI) image sensors and back-side illuminated (BSI) imagesensors.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 illustrates a cross-sectional view of some embodiments of animage sensor with a high absorption pixel sensor.

FIG. 2A illustrates a cross-sectional view of some more detailedembodiments of the image sensor of FIG. 1 with a porous semiconductorlayer.

FIG. 2B illustrates a cross-sectional view of some more detailedembodiments of the image sensor of FIG. 1 without a porous semiconductorlayer.

FIG. 3 illustrates a layout view of some embodiments of the image sensorof FIG. 1 and/or FIG. 2.

FIG. 4A illustrated a cross-sectional view of some front-sideilluminated (FSI) embodiments of the image sensor of FIG. 3.

FIG. 4B illustrated a cross-sectional view of some back-side illuminated(BSI) embodiments of the image sensor of FIG. 3.

FIGS. 5-13 illustrate a series of cross-sectional views of someembodiments of a method for manufacturing a FSI image sensor with highabsorption pixel sensors.

FIG. 14 illustrates a flowchart of some embodiments of the method ofFIGS. 5-13.

FIGS. 15-23 illustrate a series of cross-sectional views of someembodiments of a method for manufacturing a BSI image sensor with highabsorption pixel sensors.

FIG. 24 illustrates a flowchart of some embodiments of the method ofFIGS. 15-23.

DETAILED DESCRIPTION

The present disclosure provides many different embodiments, or examples,for implementing different features of this disclosure. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper”, and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice or apparatus in use or operation in addition to the orientationdepicted in the figures. The device or apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly. Even more, the terms “first”, “second”, “third”, “fourth”,and the like are merely generic identifiers and, as such, may beinterchanged in various embodiments. For example, while an element(e.g., an etch, a dielectric layer, or a substrate) may be referred toas a “first” element in some embodiments, the element may be referred toas a “second” element in other embodiments.

Some complementary metal-oxide-semiconductor (CMOS) image sensorscomprise a semiconductor substrate of monocrystalline silicon and anarray of pixel sensors arranged in the semiconductor substrate. Thepixel sensors comprise respective photodetectors buried in thesemiconductor substrate and respective pixel transistors arranged on asurface of the semiconductor substrate. The photodetectors areconfigured to absorb incident radiation and to generate an electricsignal corresponding to the incident radiation.

A challenge with the CMOS image sensors is that monocrystalline siliconhas a large energy bandgap, and hence a low absorption coefficient forhigh wavelength radiation. The high wavelength radiation includes, forexample, radiation with a wavelength greater than about 800 micrometers,such as infrared radiation. Therefore, the photodetectors have poorquantum efficiency for high wavelength radiation unless enhanced.Quantum efficiency (QE) is the fraction of incident photons whichcontribute to the electric signal.

One approach for enhancing the CMOS image sensors is to increase thethickness of the semiconductor substrate and the depth to which thephotodetectors extend into the semiconductor substrate. The higher thewavelength of incident radiation, the higher the absorption depth.However, this is difficult with existing CMOS processes and adds cost tothe manufacture of the CMOS image sensors. Further, increasing the depthto which the photodetectors extend into the semiconductor substrateincreases cross talk and die size.

In view of the foregoing, various embodiments of the present applicationare directed towards a high absorption image sensor. In someembodiments, a semiconductor substrate includes a non-poroussemiconductor layer. The non-porous semiconductor layer is along a frontside of the semiconductor substrate. A periodic structure is along aback side of the semiconductor substrate. The periodic structureincludes a plurality of protrusions defined by the semiconductorsubstrate. A high absorption layer lines the periodic structure on theback side of the semiconductor substrate. The high absorption layer is asemiconductor material with an energy bandgap less than that of thenon-porous semiconductor layer. A photodetector is in the semiconductorsubstrate and the high absorption layer.

Advantageously, by arranging the photodetector in the high absorptionlayer, the photodetector benefits from the low energy bandgap of thehigh absorption layer. Namely, the low energy bandgap renders the highabsorption layer highly absorptive of incident radiation, such that thephotodetector has high quantum efficiency. Further, the high quantumefficiency advantageously allows cost, die size, crosstalk, or acombination of the foregoing to be low when the high absorption imagesensor is employed for sensing high wavelength radiation, such asinfrared radiation, since high wavelength radiation may be sufficientlysensed without the semiconductor substrate having a large thickness.

With reference to FIG. 1, a cross-sectional view 100 of some embodimentsof an image sensor with a high absorption pixel sensor 102 is provided.The image sensor may be, for example, front-side illuminated (FSI) orback-side illuminated (BSI). Further, the image sensor may be, forexample, a CMOS image sensor, and/or an integrated circuit (IC) die orchip. As illustrated, a semiconductor substrate 104 comprises anon-porous semiconductor layer 104 n and a porous semiconductor layer104 p. The non-porous semiconductor layer 104 n is along a front-sidesurface 104 f of the semiconductor substrate 104, and the poroussemiconductor layer 104 p is along a back-side surface 104 b of thesemiconductor substrate 104 that is opposite the front-side surface 104f of the semiconductor substrate 104. In alternative embodiments, theporous semiconductor layer 104 p is omitted, such that the non-poroussemiconductor layer 104 n defines the back-side surface 104 b of thesemiconductor substrate 104.

The porous semiconductor layer 104 p is a semiconductor material with asystematic structure of pores. The pores may be, for example, micrometersized pores, nanometer sized pores, smaller sized pores, or acombination of the foregoing. The porous semiconductor layer 104 p maybe, for example, nanoporous silicon or some other type of poroussemiconductor material. The non-porous semiconductor layer 104 n is asemiconductor material without a systematic structure of pores, wherethe pores are sized as described above. For example, the non-poroussemiconductor layer 104 n may be a semiconductor material without asystematic structure of nanometer and/or micrometer sized pores. Thenon-porous semiconductor layer 104 n may be, for example,monocrystalline silicon, polycrystalline silicon, or some other type ofcrystalline semiconductor material. Further, the non-poroussemiconductor layer 104 n may, for example, have an indirect energybandgap.

In some embodiments, the semiconductor substrate 104 defines a periodicstructure 106 along the back-side surface 104 b of the semiconductorsubstrate 104. The periodic structure 106 may comprise, for example, aplurality of protrusions 106 p in a periodic pattern or array along theback-side surface 104 b of the semiconductor substrate 104. For ease ofillustration, only one of the protrusions 106 p is labeled 106 p. Theprotrusions 106 p may, for example, have a cone shape, a pyramid shape,or some other shape.

Advantageously, the porous semiconductor layer 104 p and/or the periodicstructure 106 enhance absorption of radiation 108 (e.g., light) by thesemiconductor substrate 104. For example, the porous semiconductor layer104 p may have a direct energy bandgap due to the periodic structure 106and/or pores of the porous semiconductor layer 104 p. The direct energybandgap allows the porous semiconductor layer 104 p to absorb photons108 p of the radiation 108 without dependence on phonons, such thatabsorption by the semiconductor substrate 104 may be enhanced. Asanother example, the protrusions 106 p and/or the pores of the poroussemiconductor layer 104 p increase surface area, such that absorption bythe semiconductor substrate 104 may be enhanced. As yet another example,the protrusions 106 p and/or the pores of the porous semiconductor layer104 p reduce reflection of the radiation 108 off the semiconductorsubstrate 104, such that absorption by the semiconductor substrate 104may be enhanced. Reflectance may, for example, be reduced sinceradiation 108 may enter the semiconductor substrate 104 through thepores of the porous semiconductor layer 104 p and become trapped in thesemiconductor substrate 104. Further, reflectance may, for example, bereduced since angled sidewalls of the protrusions 106 p may reduce thelikelihood of radiation reflecting away from the semiconductor substrate104.

A high absorption layer 110 lines the back-side surface 104 b of thesemiconductor substrate 104, and is sandwiched between the semiconductorsubstrate 104 and a passivation layer 112. The high absorption layer 110is a semiconductor material that has a low energy bandgap. The lowenergy bandgap may be, for example, an energy bandgap that is less thanabout 1 electron volt (eV). Further, the low energy bandgap may be, forexample, an energy bandgap that is less than an energy bandgap of thenon-porous semiconductor layer 104 n and/or an energy bandgap of theporous semiconductor layer 104 p. In some embodiments, the highabsorption layer 110 is silicon germanium, or monocrystalline silicondoped with a chalcogen (e.g., sulfur, selenium, or tellurium). Thepassivation layer 112 may be, for example, silicon dioxide, siliconnitride, some other dielectric, or a combination of the foregoing.

A photodetector 114 is in the semiconductor substrate 104 and the highabsorption layer 110, and is configured to absorb radiation 108 incidenton the photodetector 114 to generate an electric signal. The electricsignal may, for example, result from electron-hole pairs 116 generatedin response to absorbing photons 108 p of the radiation 108. Thephotodetector 114 may be, for example, a photodiode. Further, thephotodetector 114 may, for example, comprise an n-type region (notshown) and a p-type region (not shown) that define a photo junction(e.g., a PN or PIN junction).

Advantageously, by arranging the photodetector 114 in the semiconductorsubstrate 104 and the high absorption layer 110, the photodetector 114is highly absorptive of radiation 108, including infrared radiation(e.g., radiation with a wavelength greater than about 800 micrometers).Namely, the high absorption layer 110 has a low energy bandgap, andhence a high absorption coefficient, such that the high absorption layer110, and hence the photodetector 114, are highly absorptive of radiation108. As a result, the photodetector 114 advantageously has high quantumefficiency. Even more, quantum efficiency of the photodetector 114 isfurther enhanced by the porous semiconductor layer 104 p and/or theperiodic structure 106, since the porous semiconductor layer 104 pand/or the periodic structure 106 enhance absorption of radiation 108 bythe semiconductor substrate 104.

With reference to FIG. 2A, a cross-sectional view 200A of some moredetailed embodiments of the image sensor of FIG. 1 is provided. Asillustrated, a semiconductor substrate 104 comprises a non-poroussemiconductor layer 104 n and a porous semiconductor layer 104 p. Thenon-porous semiconductor layer 104 n is along a front-side surface 104 fof the semiconductor substrate 104, and the porous semiconductor layer104 p is along a back-side surface 104 b of the semiconductor substrate104 that is opposite the front-side surface 104 f of the semiconductorsubstrate 104.

In some embodiments, the semiconductor substrate 104 defines a periodicstructure 106 along the back-side surface 104 b of the semiconductorsubstrate 104. The periodic structure 106 may comprise, for example, aplurality of protrusions 106 p in a periodic pattern or array along theback-side surface 104 b. In some embodiments, pore size of the poroussemiconductor layer 104 p increases or decreases gradually from tips ofthe protrusions 106 p to an interface between the non-poroussemiconductor layer 104 n and the porous semiconductor layer 104 p. Forexample, pore size may decrease gradually from an average size of about40 nanometers at the tips to an average size of about 8 nanometers atthe interface.

In some embodiments, the protrusions 106 p define a saw-toothed profile.In some embodiments, the protrusions 106 p have a pitch P of about0.01-8.0 micrometers, about 0.2-5.0 micrometers, about 1.0-3.0micrometers, or a combination of the foregoing. In some embodiments, theprotrusions 106 p have a height H of about 0.2-20.0 micrometers, about1.0-15.0 micrometers, about 5.0-10.0 micrometers, or a combination ofthe foregoing. In some embodiments, the protrusions 106 p have taperedsidewalls meeting at tips. For example, some or all of the protrusions106 p may each be cone shaped or pyramid shaped (e.g., have an n-sidedbase, wherein n=3, 4, 5, 6, etc.). In other embodiments, the protrusions106 p have another shape. For example, some or all of the protrusionsmay be cylinder shaped.

Advantageously, the porous semiconductor layer 104 p and/or the periodicstructure 106 may enhance absorption of radiation by the semiconductorsubstrate 104. For example, the porous semiconductor layer 104 p mayhave a direct energy bandgap that enhances absorption. As anotherexample, the porous semiconductor layer 104 p and/or the periodicstructure 106 may increase surface area for enhanced absorption. As yetanother example, the porous semiconductor layer 104 p may reducereflection off the semiconductor substrate 104 for enhanced absorption.

A high absorption layer 110 lines the back-side surface 104 b of thesemiconductor substrate 104, and is sandwiched between the semiconductorsubstrate 104 and a passivation layer 112. In some embodiments, the highabsorption layer 110 conformally lines the back-side surface 104 b ofthe semiconductor substrate 104. The high absorption layer 110 is asemiconductor material that has a low energy bandgap. The low energybandgap may be, for example, less than about 1.00 eV, 0.80 eV, 0.60 eV,or 0.40 eV. Further, the low energy bandgap may be, for example, lessthan an energy bandgap of the non-porous semiconductor layer 104 nand/or an energy bandgap of the porous semiconductor layer 104 p. Forexample, the high absorption layer 110 may have an energy bandgap lessthan about 1.0 eV, whereas the non-porous semiconductor layer 104 n mayhave an energy bandgap greater than about 1.1 eV, 1.5 eV, 2.0 eV, or 5.0eV.

In some embodiments, the high absorption layer 110 has a direct energybandgap, such that absorption of incident radiation is advantageouslynot dependent on phonons. In some embodiments, the high absorption layer110 has a thickness T of about 10-5000 angstroms, 100-4000 angstroms,1000-3000 angstroms, or a combination of the foregoing. For example, thehigh absorption layer 110 may have a thickness T of about 500 angstromsor about 3000 angstroms. In some embodiments, the high absorption layer110 is silicon germanium or monocrystalline silicon, and/or is dopedwith a chalcogen. For example, the high absorption layer 110 may besilicon germanium devoid of a chalcogen, or may be monocrystallinesilicon doped with a chalcogen. As another example, the high absorptionlayer 110 may be Si_(0.8)Ge_(0.2) with a thickness T of about 500angstroms. In some embodiments where the high absorption layer 110 isdoped with a chalcogen, the high absorption layer 110 is doped in excessof the solubility limit of the chalcogen (i.e., is supersaturated) toadvantageously facilitate sub-band gap absorption of incident radiation(e.g., radiation with wavelengths of about 1.0-2.5 micrometers). Thechalcogen may be, for example, sulfur (S), selenium (Se), tellurium(Te), or a combination of the foregoing. In some embodiments, the highabsorption layer 110 interfaces with the semiconductor substrate 104 todefine a heterojunction at the interface.

The passivation layer 112 underlies the high absorption layer 110 andmay be, for example, silicon dioxide, silicon nitride, a high κdielectric, some other dielectric, or a combination of the foregoing. Ahigh κ dielectric is a dielectric with a dielectric constant κ greaterthan about 3.9, 5, 10, or 20. In some embodiments, the passivation layer112 has an inner surface 112 i (e.g., an upper or top surface)conforming to the back-side surface 104 b of the semiconductor substrate104 through the high absorption layer 110. Further, in some embodiments,the passivation layer 112 has an outer surface 112 o (e.g., a lower orbottom surface) that is planar.

A high absorption pixel sensor 102 is in the semiconductor substrate 104and the high absorption layer 110, and comprises a photodetector 114.The photodetector 114 is configured to to absorb radiation incident onthe photodetector 114 and to generate an electric signal from theabsorbed radiation. For example, the electric signal may result fromelectron-hole pairs generated in response to absorbing photons of theradiation. The photodetector 114 comprises a photo junction 202, as wellas a first doping-type region 114 a and a second doping-type region 114b. The photo junction 202 is between, and at least partially defined by,the first and second doping-type regions 114 a, 114 b. The photojunction 202 may be, for example, a PN junction, a PIN junction, or acombination of the foregoing, and/or may be, for example, aheterojunction, a homojunction, or a combination of the foregoing.

The first doping-type region 114 a is in the non-porous semiconductorlayer 104 n and, in some embodiments, is in the porous semiconductorlayer 104 p. Further, the first doping-type region 114 a borders thesecond doping-type region 114 b and has an opposite doping type as thesecond doping-type region 114 b. For example, the first doping-typeregion 114 a may be n-type and the second doping-type region 114 b maybe p-type, or vice versa. The first doping-type region 114 a isconfigured to accumulate charge (e.g., electrons) from electron-holepairs generated in response to radiation incident on the photodetector114. For example, when an electron-hole pair is generated in the firstor second doping-type region 114 a, 114 b, charge (e.g., an electron) ofthe electron-hole pair may diffuse and/or drift to the first doping-typeregion 114 a due to an electric field generated by the photo junction202.

The second doping-type region 114 b is in the high absorption layer 110and, in some embodiments, the semiconductor substrate 104. Further, insome embodiments, the second doping-type region 114 b extendscontinuously with a single doping type along sidewalls of the firstdoping-type region 114 a, and along an underside of the firstdoping-type region 114 a, so as to cup first doping-type region 114 a.

Advantageously, by arranging the photodetector 114 in the semiconductorsubstrate 104 and the high absorption layer 110, the photodetector 114is highly absorptive of incident radiation, including infraredradiation. Namely, the high absorption layer 110 has a low energybandgap, and hence a high absorption coefficient, such that the highabsorption layer 110, and hence the photodetector 114, are highlyabsorptive of incident radiation. As a result, the photodetector 114advantageously has high quantum efficiency. Further, the high quantumefficiency advantageously allows cost, die size, crosstalk, or acombination of the foregoing to be low when the image sensor is employedfor sensing high wavelength radiation, such as infrared radiation, sincehigh wavelength radiation may be sufficiently sensed without thesemiconductor substrate 104 having a large thickness. Even more, quantumefficiency of the photodetector 114 is further enhanced by the poroussemiconductor layer 104 p and/or the periodic structure 106, since theporous semiconductor layer 104 p and/or the periodic structure 106enhance absorption of radiation by the semiconductor substrate 104.

In some embodiments, quantum efficiency of the photodetector 114 isfurther enhanced by a heterojunction at an interface betweensemiconductor substrate 104 and the high absorption layer 110. Forexample, where the high absorption layer 110 is silicon germanium andthe porous semiconductor layer 104 p is porous silicon, a heterojunctionbetween the high absorption layer 110 and the porous semiconductor layer104 p may increase quantum efficiency. However, a heterojunctionincreases quantum efficiency at the cost of an increase in dark current,which may lead to white pixels. Namely, a mismatch in lattice constantsat the heterojunction increases strain at the heterojunction, whichincreases dark current.

To reduce strain, and hence dark current, at the heterojunction, acarbon doped region 204 is at the heterojunction in some embodiments.The carbon doped region 204 buffers and reduces strain, thereby reducingdark current. The carbon doped region 204 may be, for example, a regionof the high absorption layer 110 that has an elevated concentration ofcarbon relative to a remainder of the high absorption layer 110.Alternatively, the carbon doped region 204 may be, for example, a regionof the semiconductor substrate 104 that has an elevated concentration ofthe carbon relative to a remainder of the semiconductor substrate 104.In some embodiments, the carbon doped region 204 has a dopingconcentration gradually increasing or decreasing from the highabsorption layer 110 to the semiconductor substrate 104.

In some embodiments, quantum efficiency of the photodetector 114 iscontrolled by varying properties of the high absorption layer 110.However, consideration should be given to a trade-off between darkcurrent and quantum efficiency. For example, increasing the thickness Tof the high absorption layer 110 advantageously increases quantumefficiency, but does so at the cost of increased dark current. Asanother example, increasing the amount of germanium and/or cholcogens inthe high absorption layer 110 advantageously increases quantumefficiency, but does so at the cost of increased dark current.

The high absorption pixel sensor 102 further comprises one or more pixeltransistors on the front-side surface 104 f of the semiconductorsubstrate 104 to facilitate readout of the photodetector 114. Forexample, the one or more pixel transistors may comprise a transfertransistor 206, a source-follower transistor (not shown), a row selecttransistor (not shown), a reset transistor (not shown), some other pixeltransistor, or a combination of the foregoing. The transfer transistor206 is configured to selectively transfer charge accumulated in thefirst doping-type region 114 a out of the first doping-type region 114 afor readout. In some embodiments, the transfer transistor 206 comprisesa first source/drain region, a gate electrode 208, a gate dielectriclayer 210, and a second source/drain region. The first source/drainregion may be, for example, the first doping-type region 114 a. Thesecond source/drain region may be, for example, a floating diffusionnode (FDN) 212 to which the transfer transistor 206 transfers chargefrom first doping-type region 114 a.

The gate electrode 208 is between and borders the first and secondsource/drain regions, and is spaced over the semiconductor substrate 104by the gate dielectric layer 210. The gate electrode 208 may be, forexample, doped polysilicon, copper, aluminum copper, some otherconductive material, or a combination of the foregoing. The gatedielectric layer 210 may be, for example, silicon dioxide, a high κdielectric, some other dielectric, or a combination of the foregoing.The first and second source/drain regions are doped regions of thesemiconductor substrate 104 having opposite doping types as surroundingregions of the semiconductor substrate 104. For example, the first andsecond source/drain regions may be n or p type.

In some embodiments, an isolation structure 214 extends into thefront-side surface 104 f of the semiconductor substrate 104, andlaterally surrounds the high absorption pixel sensor 102, toelectrically isolate the high absorption pixel sensor 102 fromsurrounding devices (not shown). Such surrounding devices may include,for example, other pixel sensors, logic devices, or memory devices.Further, in some of such embodiments, the isolation structure 214comprises a ring-shaped layout. The isolation structure 214 may, forexample, be a shallow trench isolation (STI) region, a deep trenchisolation (DTI) region, an implant isolation region, or a combination ofthe foregoing.

An interconnect structure 216 is over the semiconductor substrate 104and the one or more pixel transistors (e.g., the transfer transistor206). Further, in some embodiments, the interconnect structure 216 isarranged over the isolation structure 214. The interconnect structure216 comprises an interlayer dielectric (ILD) layer 218, a plurality ofwiring layers 220, and a plurality of vias 222. For ease ofillustration, only one of the wiring layers 220 is labeled 220 and onlyone of the vias 222 is labeled 222. The ILD layer 218 may be, forexample, silicon dioxide, silicon nitride, a low κ dielectric (e.g.,fluorosilicate glass (FSG)), some other dielectric, or a combination ofthe foregoing. A low κ dielectric is a dielectric with a dielectricconstant κ less than about 3.9, 3.0, 2.0, or 1.0.

The wiring layers 220 are alternatingly stacked with the vias 222 in theILD layer 218. The vias 222 electrically couple the wiring layers 220together, and further electrically couple a wiring layer (notindividually labeled) closest to the semiconductor substrate 104 todevices (e.g., the transfer transistor 206) on the front-side surface104 f of semiconductor substrate 104. The wiring layers 220 are aconductive material, such as, for example, aluminum copper, copper,aluminum, some other conductive material, or a combination of theforegoing, and the vias 222 are a conductive material, such as, forexample, copper, tungsten, some other conductive material, or acombination of the foregoing.

With reference to FIG. 2B, a variant of FIG. 2A is provided in which theporous semiconductor layer 104 p of FIG. 2A is omitted. In suchembodiments, the high absorption layer 110 contacts the non-poroussemiconductor layer 104 n. Further, in some embodiments, the non-poroussemiconductor layer 104 n defines the periodic structure 106.

While FIGS. 2A and 2B illustrate a specific embodiment of thephotodetector 114, it should be appreciated that other embodiments ofthe photodetector 114 are amenable. For example, in embodiments wherethe photo junction 202 is or otherwise comprises a PIN junction, anintrinsic semiconductor region (not shown) may be arranged between andcontacting the first doping-type region 114 a and the second doping-typeregion 114 b. The intrinsic semiconductor region may be, for example,monocrystalline silicon or silicon germanium.

With reference to FIG. 3, a layout view 300 of some embodiments of theimage sensor of FIG. 1, FIG. 2A, or FIG. 2B is provided. As illustrated,a logic region 302 laterally surrounds a pixel sensor array 304. Thelogic region 302 comprises, for example, logic and/or memory devices(not shown) configured to read and/or store data generated by the pixelsensor array 304 in response to incident radiation.

The pixel sensor array 304 is made up of a plurality of high absorptionpixel sensors 102 arranged in X rows and Y columns. X and Y are integervalues greater than zero and may be, for example, the same or different.For example, X and Y may both be 128, 256, 512, 1024, 2048, 4096, or Xand Y may respectively be 768 and 1024, 1024 and 2048, 256 and 1024, 512and 128, or 4096 and 1024. For ease of illustration, only one of thehigh absorption pixel sensors 102 is labeled 102. The high absorptionpixel sensors 102 are individually configured as described with regardto FIG. 1, FIG. 2A, or FIG. 2B.

An isolation structure 214 laterally surrounds the pixel sensor array304 to electrically isolate the pixel sensor array 304 from the logicregion 302. Further, the isolation structure 214 laterally surroundseach of the high absorption pixel sensors 102 to electrically isolatethe high absorption pixel sensors 102 from each other.

With reference to FIGS. 4A and 4B, cross-sectional views 400A, 400B ofvarious embodiments of the image sensor of FIG. 3 are provided. Thecross-sectional views 400A, 400B may, for example, be taken along line Ain FIG. 3.

As illustrated by the cross-sectional view 400A of FIG. 4A, the imagesensor is FSI, and comprises a plurality of high absorption pixelsensors 102 a-102 c. The high absorption pixel sensors 102 a-102 c areeach configured according the high absorption pixel sensor 102 of FIG.1, FIG. 2A, or FIG. 2B, and are each configured to receive radiation 108through an interconnect structure 216 covering the high absorption pixelsensors 102 a-102 c. The high absorption pixel sensors 102 a-102 c eachcomprise a photodetector 114 and, in some embodiments, one or more pixeltransistors. For example, the high absorption pixel sensors 102 a-102 cmay each comprise a transfer transistor 206. For ease of illustration,only one of the photodetectors 114 is labeled 114, and only one of thetransfer transistors 206 is labeled 206.

The interconnect structure 216 comprises an interlayer dielectric (ILD)layer 218, a plurality of wiring layers 220, and a plurality of vias222. For ease of illustration, only one of the wiring layers 220 islabeled 220, and only one of the vias 222 is labeled 222. The wiringlayers 220 are alternatingly stacked with the vias 222 in the ILD layer218. In some embodiments, the wiring layers 220 and the vias 222 arelimited to sides of the photodetectors 114 (i.e., are not directly overthe photodetectors 114) so radiation is not blocked from impinging onthe photodetectors 114.

The photodetectors 114 are in a semiconductor substrate 104 and a highabsorption layer 110 stacked under the interconnect structure 216.Further, in some embodiments, the photodetectors 114 are electricallyisolated by an isolation structure 214 (e.g., an STI region) between thehigh absorption pixel sensors 102 a-102 c. The photodetectors 114 eachcomprise a first doping-type region 114 a in the semiconductor substrate104, a second doping-type region 114 b in the high absorption layer 110,and a photo junction 202 at least partially defined by the first andsecond doping-type regions 114 a, 114 b. For ease of illustration, onlyone of the first doping-type regions 114 a is labeled 114 a, only one ofthe second doping-type regions 114 b is labeled 114 b, and only one ofthe photo junctions 202 is labeled 202.

The transfer transistors 206 are on a front-side surface 104 f of thesemiconductor substrate 104, between the semiconductor substrate 104 andthe interconnect structure 216. The transfer transistors 206 eachcomprise a gate electrode 208 and a gate dielectric layer 210 spacingthe gate electrode 208 from the semiconductor substrate 104. Further,the transfer transistors 206 each comprise a first source/drain regionand a second source/drain region respectively bordering oppositesidewalls of the gate electrode 208. The first source/drain region maybe, for example, a first doping-type region 114 a of a respectivephotodetector 114, and/or the second source/drain region may be, forexample, a FDN 212. For ease of illustration, only one of the gateelectrodes 208 is labeled 208, only one of the FDNs 212 is labeled 212,and only one of the gate dielectric layers 210 is labeled 210.

The semiconductor substrate 104 overlies the high absorption layer 110,such that the high absorption layer 110 is on a back-side surface 104 bof the semiconductor substrate 104. The semiconductor substrate 104comprises a non-porous semiconductor layer 104 n and a poroussemiconductor layer 104 p underlying the non-porous semiconductor layer104 n. In alternative embodiments, the porous semiconductor layer 104 pis omitted, such that the non-porous semiconductor layer 104 ncompletely fills the space presently occupied by the poroussemiconductor layer 104 p in FIG. 4A.

In some embodiments, the semiconductor substrate 104 defines a periodicstructure 106 along the back-side surface 104 b of the semiconductorsubstrate 104. The periodic structure 106 may comprise, for example, aplurality of protrusions 106 p in a periodic pattern or array along theback-side surface 104 b of the semiconductor substrate 104. For ease ofillustration, only one of the protrusions 106 p is labeled 106 p.

The high absorption layer 110 has a low energy band gap. For example,the high absorption layer 110 may have an energy bandgap less than about1.0 eV, 0.8 eV, or 0.5 eV. Due to the low energy bandgap, the highabsorption layer 110 has a high absorption coefficient. Further, thephotodetectors 114, which are partially in the high absorption layer110, have high quantum efficiency. In some embodiments, the highabsorption layer 110 is silicon germanium, or monocrystalline silicondoped with a chalcogen. Further, in some embodiments, the highabsorption layer 110 has a doping type opposite that of the firstdoping-type regions 114 a.

A passivation layer 112 underlies the high absorption layer 110 and, insome embodiments, a carrier substrate 402 underlies and is bonded to thepassivation layer 112. The carrier substrate 402 may be, for example, abulk semiconductor substrate and/or monocrystalline silicon. Further,the carrier substrate 402 may be, for example, a semiconductor wafer(e.g., a 450 millimeter silicon wafer).

In some embodiments, color filters 404 respectively cover the highabsorption pixel sensors 102 a-102 c. The color filters 404 areconfigured to pass assigned wavelengths of radiation, while blockingunassigned wavelengths of radiation. For example, a color filter may beconfigured to pass red wavelengths of radiation, while blocking bluewavelengths of radiation, whereas another color filter may be configuredto pass blue wavelengths of radiation, while blocking red wavelengths ofradiation. In some embodiments, the color filters 404 are respectivelycovered by micro-lenses 406 configured to focus incident radiation onrespective photodetectors of the high absorption pixel sensors 102 a-102c. For ease of illustration, only one of the color filters 404 islabeled 404, and only one of the micro-lenses 406 is labeled 406.

As illustrated by the cross-sectional view 400B of FIG. 4B, a variant ofFIG. 4A is provided in which the image sensor is BSI. In contrast withFIG. 4A, the high absorption pixel sensors 102 a-102 c are over theinterconnect structure 216. Further, the high absorption pixel sensors102 a-102 c are each configured to receive radiation 108 through theback-side surface 104 b of the semiconductor substrate 104. In someembodiments, the porous semiconductor layer 104 p and/or the periodicstructure 106 reduce reflection of the radiation 108 off thesemiconductor substrate 104, such that the absorption may be enhanced.Reflectance may, for example, be reduced since radiation 108 may enterthe semiconductor substrate 104 through pores in the poroussemiconductor layer 104 p and become trapped in the semiconductorsubstrate 104. Further, reflectance may, for example, be reduced sinceangled sidewalls of the protrusions 106 p of the periodic structure 106may reduce the likelihood of radiation reflecting away from thesemiconductor substrate 104.

With reference to FIGS. 5-13, a series of cross-sectional views 500-1300of some embodiments of a method for manufacturing a FSI image sensorwith high absorption pixel sensors is provided. For example, the methodmay be employed to manufacture the image sensor of FIG. 4A.

As illustrated by the cross-sectional view 500 of FIG. 5, asemiconductor substrate 104 is provided. In some embodiments, thesemiconductor substrate 104 comprises a non-porous semiconductor layer104 n and a porous semiconductor layer 104 p. The non-poroussemiconductor layer 104 n is along a front-side surface 104 f of thesemiconductor substrate 104, and the porous semiconductor layer 104 p isalong a back-side surface 104 b of the semiconductor substrate 104 thatis opposite the front-side surface 104 f of the semiconductor substrate104. In alternative embodiments, the porous semiconductor layer 104 p isomitted. In some of such embodiments, the non-porous semiconductor layer104 n is along both the front-side surface 104 f of the semiconductorsubstrate 104 and the back-side surface 104 b of the semiconductorsubstrate 104. Further, in some of such embodiments, the non-poroussemiconductor layer 104 n defines the whole of the semiconductorsubstrate 104.

The porous semiconductor layer 104 p is a semiconductor material with asystematic structure of pores. The pores may be, for example, micrometersized pores, nanometer sized pores, smaller sized pores, or acombination of the foregoing. The porous semiconductor layer 104 p maybe, for example, nanoporous silicon. The non-porous semiconductor layer104 n is a semiconductor material without a systematic structure ofpores, where the pores are sized as described above. For example, thenon-porous semiconductor layer 104 n may be a semiconductor materialwithout a systematic structure of nanometer and/or micrometer sizedpores. The non-porous semiconductor layer 104 n may be, for example,monocrystalline silicon or some other type of crystalline semiconductormaterial.

In some embodiments, the process for providing the semiconductorsubstrate 104 comprises performing an etch into the non-poroussemiconductor layer 104 n to form the porous semiconductor layer 104 pfrom a portion of the non-porous semiconductor layer 104 n. The etchmay, for example, be performed by applying a solution 502 ofhydrofluoric acid to the non-porous semiconductor layer 104 n, andsubsequently activating the hydrofluoric acid while the solution 502 ison the non-porous semiconductor layer 104 n. The hydrofluoric acid may,for example, be activated: electrochemically by applying a sufficientvoltage to the solution 502 (anodic etching); electrochemically byadding an oxidant with a sufficient standard electrode potential to thesolution 502 (stain etching); or photoelectrochemically by irradiatingthe solution 502 with a laser or lamp with a sufficiently shortwavelength to excite electrons in the solution 502 and/or the non-poroussemiconductor layer 104 n to the conduction band. In some embodiments,the solution 502 further includes a ferric nitride.

As illustrated by the cross-sectional view 600 of FIG. 6, a periodicstructure 106 is formed in the back-side surface 104 b of thesemiconductor substrate 104. In some embodiments where the poroussemiconductor layer 104 p is present, the periodic structure 106 isformed directly in the porous semiconductor layer 104 p. In someembodiments where the porous semiconductor layer 104 p is omitted, theperiodic structure 106 is formed directly in the non-poroussemiconductor layer 104 n. In some embodiments, the periodic structure106 comprises a plurality of protrusions 106 p in a periodic pattern.For ease of illustration, only one of the protrusions 106 p is labeled106 p. The periodic pattern may, for example, be a two-dimensional arrayof protrusions in a plurality of rows and a plurality of columns.

In some embodiments, the process for forming the periodic structure 106comprises selectively dry etching the back-side surface 104 b of asemiconductor substrate 104. For example, a photoresist layer 602 may beformed on the back-side surface 104 b, and patterned with the periodicpattern of the protrusions 106 p. A dry etchant 604 may then be appliedto the back-side surface 104 b with the photoresist layer 602 in placeto form the protrusions 106 p, and the photoresist layer 602 may besubsequently stripped. In some embodiments, the process for forming theperiodic structure 106 further comprises wet etching the back-sidesurface 104 b of the semiconductor substrate 104 to remove damage to thesemiconductor substrate 104 from the selective dry etching. For example,a wet etchant (not shown) may be applied to the back-side surface 104 bbefore or after the photoresist layer 602 is stripped. The wet etchantmay be, for example, tetramethylammonium hydroxide (TMAH).

The damage to the semiconductor substrate 104 may lead to electron trapsalong the back-side surface 104 b of a semiconductor substrate 104.Further, the electron traps may, in turn, lead to leakage current, darkcurrent and white pixels, or a combination of the foregoing sincephotodetectors formed hereafter are formed in the semiconductorsubstrate 104, along the back-side surface 104 b. Therefore, the wetetch advantageously removes the electron traps.

As illustrated by the cross-sectional view 700 of FIG. 7, a highabsorption layer 110 is formed on the back-side surface 104 b of asemiconductor substrate 104. In some embodiments where the poroussemiconductor layer 104 p is present, the high absorption layer 110 isformed in direct contact with the porous semiconductor layer 104 p. Insome embodiments where the porous semiconductor layer 104 p is omitted,the high absorption layer 110 is formed in direct contact with thenon-porous semiconductor layer 104 n. In some embodiments, the highabsorption layer 110 is formed conformally. In some embodiments, thehigh absorption layer 110 is formed with the same doping type as theporous semiconductor layer 104 p and/or the non-porous semiconductorlayer 104 n. In some embodiments, the high absorption layer 110 isformed of a semiconductor material with a direct bandgap, and/or isformed with a low band gap. A low band gap may be, for example, a bandgap that is less than about 1.0 eV, 0.8 eV, or 0.5 eV, and/or that isless than a bandgap of the non-porous semiconductor layer 104 n.

In some embodiments, the high absorption layer 110 is formed as a dopedregion of the semiconductor substrate 104. In some of such embodiments,the high absorption layer 110 is doped with a chalcogen, such as, forexample, S, Se, Te, or a combination. The doping may, for example, beperformed by ion implantation into the back-side surface 104 b of thesemiconductor substrate 104. Further, in some embodiments where the highabsorption layer 110 is doped with the chalcogen, the high absorptionlayer 110 is doped in excess of the solubility limit (i.e., issupersaturated) to advantageously facilitate sub-band gap absorption ofphotons. The supersaturation may, for example, be performed by ionimplantation into the back-side surface 104 b, followed by pulsed lasermelting (e.g., 1-7, 2-6, 3-5, or 4 laser shots).

In other embodiments, the high absorption layer 110 is an epitaxiallayer. In some of the epitaxial embodiments, the high absorption layer110 is silicon germanium, or alternatively silicon (e.g., amonocrystalline silicon) doped with a chalcogen. For example, theepitaxial layer may be silicon doped with S, Se, Te, or a combination ofthe foregoing. Further, in some of the epitaxial embodiments, the highabsorption layer 110 may be supersaturated with a chalcogen as describedabove. The epitaxy may, for example, be performed by, for example,chemical vapor deposition, physical vapor deposition, or molecular beamepitaxy, and/or the doping may, for example, be performed by ionimplantation.

As illustrated by cross-sectional view 800 of FIG. 8, a passivationlayer 112 is formed over the high absorption layer 110. In someembodiments, the passivation layer 112 is formed with an inner surface112 i conforming to the back-side surface 104 b of the semiconductorsubstrate 104 and/or to the high absorption layer 110. For example, thepassivation layer 112 may comprise a plurality of depressions 112 drespectively receiving the protrusions 106 p of the periodic structure106. Further, in some embodiments, the passivation layer 112 is formedwith an outer surface 112 o that is planar and that is on an oppositeside of the passivation layer 112 as the high absorption layer 110.

The process for forming the passivation layer 112 may comprise, forexample, depositing or growing the passivation layer 112. The depositingor growing may be performed by, for example, thermal oxidation, chemicalor physical vapor deposition, sputtering, some other deposition orgrowth process, or a combination of the foregoing. Further, the processfor forming the passivation layer 112 may comprise, for example,performing a planarization into the outer surface 112 o of thepassivation layer 112. The planarization may, for example, be performedby a chemical mechanical polish (CMP).

As illustrated by the cross-sectional view 900 of FIG. 9, in someembodiments, the passivation layer 112 is bonded to a carrier substrate402 at the outer surface 112 o of the passivation layer 112. The carriersubstrate 402 may be, for example, monocrystalline silicon or some othersemiconductor material. In some embodiments, the bonding processcomprises a fusion bonding process.

As illustrated by the cross-sectional view 1000 of FIG. 10, thestructure of FIG. 9 is flipped vertically and, in some embodiments, thesemiconductor substrate 104 is thinned to reduce a thickness of thesemiconductor substrate 104 from a first thickness T₁ to a secondthickness T₂. The second thickness T₂ may, for example, be less than athird thickness T₃ of the carrier substrate 402. In some embodiments,the semiconductor substrate 104 is thinned by performing an etch intothe front-side surface 104 f of the semiconductor substrate 104 and/orby performing a planarization into the front-side surface 104 f of thesemiconductor substrate 104. The planarization may, for example, beperformed by a CMP.

As illustrated by the cross-sectional view 1100 of FIG. 11, a pluralityof high absorption pixel sensors 102 a-102 c are formed on thefront-side surface 104 f of the semiconductor substrate 104. Each of thehigh absorption pixel sensors 102 a-102 c is formed with a photodetector114 and, in some embodiments, one or more pixel transistors. The one ormore pixel transistors for a pixel sensor may comprise, for example, atransfer transistor 206, a source follower transistor (not shown), areset transistor (not shown), a row select transistor (not shown), or acombination of the foregoing. For ease of illustration, only one of thephotodetectors 114 is labeled 114, and only one of the transfertransistors 206 is labeled 206.

The photodetectors 114 are each formed with a first doping-type region114 a, a second doping-type region 114 b, and a photo junction 202. Forease of illustration, only one of first doping-type regions 114 a islabeled 114 a, only one of the second doping-type regions 114 b islabeled 114 b, and only one of the photo junctions 202 is labeled 202.The first doping-type regions 114 a are formed in the semiconductorsubstrate 104 and are formed with an opposite doping type as the seconddoping-type regions 114 b. The second doping-type regions 114 b areformed bordering the first doping-type regions 114 a, and are formed inthe high absorption layer 110 and, in some embodiments, thesemiconductor substrate 104. The photo junctions 202 are formed as PNand/or PIN junctions and are at least partially defined by the firstdoping-type regions 114 a and the second doping-type regions 114 b.

In some embodiments, the semiconductor substrate 104 and/or the highabsorption layer 110 initially have a single, first doping type, suchthat the photodetectors 114 may be formed by forming the firstdoping-type regions 114 a in the semiconductor substrate 104 with asecond doping type opposite the first doping type. The first doping-typeregions 114 a may, for example, be formed by selectively implantingdopants into the semiconductor substrate 104.

The transfer transistors 206 each comprise a gate electrode 208, a gatedielectric layer 210, a first source/drain region, and a secondsource/drain region. The first source/drain region may be, for example,a respective one of the first doping-type regions 114 a. The secondsource/drain region may be, for example, a FDN 212. For ease ofillustration, only one of the gate electrodes 208 is labeled 208, onlyone of the gate dielectric layers 210 is labeled 210, and only one ofthe FDNs 212 is labeled 212. The gate electrodes 208 are spaced over thesemiconductor substrate 104 by the gate dielectric layers 210, and eachhave opposite sides respectively bordering a first source/drain regionand a second source/drain region.

In some embodiments, the process for forming the transfer transistors206 (or other pixel transistors) is performed before forming thephotodetectors 114. Further, in some embodiments, the process forforming the transfer transistors 206 comprises forming a dielectriclayer and an electrode layer stacked over the semiconductor substrate104. The dielectric layer may be formed by, for example, thermaloxidation, vapor deposition, sputtering, some other deposition or growthprocess, or a combination of the foregoing. The electrode layer may beformed by, for example, electrochemical plating, vapor deposition,sputtering, some other deposition or growth process, or a combination ofthe foregoing. Further, in some embodiments, the process for forming thetransfer transistors 206 comprises performing a selective etch into thedielectric layer and the electrode layer to define the gate electrodes208 and the gate dielectric layers 210. The selective etching may, forexample, be performed using photolithography. Even more, in someembodiments, the process for forming the transfer transistors 206comprises selectively implanting dopants into the semiconductorsubstrate 104, thereby defining the first doping-type regions 114 a ofthe photodetectors 114 and the FDNs 212.

Also illustrated by the cross-sectional view 1100 of FIG. 11, anisolation structure 214 is formed between regions of the semiconductorsubstrate 104 corresponding to the high absorption pixel sensors 102a-102 c. The isolation structure 214 may, for example, be formed as anSTI region, a DTI region, an implant isolation region, or a combinationof the foregoing. In some embodiments, the process for forming theisolation structure 214 comprises selectively etching the semiconductorsubstrate 104 to form trenches, and subsequently forming one or moredielectric materials filling the trenches. Further, in some embodiments,the isolation structure 214 is formed prior to forming the highabsorption pixel sensors 102 a-102 c.

As illustrated by the cross-sectional view 1200 of FIG. 12, aninterconnect structure 216 is formed over the high absorption pixelsensors 102 a-102 c and the semiconductor substrate 104. Theinterconnect structure 216 comprises an ILD layer 218, a plurality ofwiring layers 220, and a plurality of vias 222. For ease ofillustration, only one of the wiring layers 220 is labeled 220, and onlyone of the vias 222 is labeled 222. The wiring layers 220 arealternatingly stacked with the vias 222 in the ILD layer 218.

In some embodiments, the process for forming the interconnect structure216 comprises repeatedly forming an ILD sublayer (i.e., a sublayer ofthe ILD layer 218) over the semiconductor substrate 104, performing aplanarization into an upper or top surface of the ILD sublayer,selectively etching the ILD sublayer to form a via opening and/or awiring opening, and filling the via opening and/or the wiring openingwith a conductive material. The ILD sublayer may, for example, be formedby thermal oxidation, chemical or physical vapor deposition, sputtering,some other growth or deposition process, or a combination of theforegoing. The planarization may, for example, be performed by a CMP.The selective etching may, for example, be performed usingphotolithography. The filling may, for example, be performed by chemicalor physical vapor deposition, electroplating, electro-less plating, someother growth or deposition process, or a combination of the foregoing.In some embodiments, the process for forming the interconnect structure216 comprises repeatedly performing a dual-damascene-like process or asingle-damascene-like process to form the wiring layers 220 and the vias222. The dual-damascene-like and single-damascene-like processes arerespectively dual-damascene and single damascene processes that are notrestricted to copper.

As illustrated by the cross-sectional view 1300 of FIG. 13, a pluralityof color filters 404 are formed respectively over the high absorptionpixel sensors 102 a-102 c. For ease of illustration, only one of thecolor filters 404 is labeled 404. In some embodiments, the color filters404 are formed by forming a color filter layer assigned wavelengths ofradiation (e.g., red wavelengths), patterning the color filter layer,and then repeating the foregoing for different wavelengths of radiation(e.g., blue wavelengths).

Also illustrated by the cross-sectional view 1300 of FIG. 13, aplurality of micro-lenses 406 are formed respectively over the colorfilters 404. In some embodiments, the process for forming themicro-lenses 406 comprises forming a micro-lens layer over the colorfilters 404, and subsequently forming micro-lens templates individual tothe micro-lenses 406 over the micro-lens layer. The micro-lens layermay, for example, be formed by a spin-on process or a depositionprocess. The micro-lens templates may, for example, be formed bydepositing (e.g., by sputtering) a photoresist layer over the micro-lenslayer, patterning the photoresist layer with a layout of themicro-lenses to form the micro-lens templates, and performing a reflowoperation to curve upper or top surfaces of the micro-lens templates. Insome embodiments, the process further comprises etching the micro-lenslayer with the micro-lens templates in place, and stripping themicro-lens templates.

With reference to FIG. 14, a flowchart 1400 of some embodiments of themethod of FIGS. 5-13 is provided.

At 1402, a semiconductor substrate is provided, where the semiconductorsubstrate comprises a non-porous semiconductor layer along a front sideof the semiconductor substrate. In some embodiments, the semiconductorsubstrate further comprises a porous semiconductor layer along a backside of the semiconductor substrate. See, for example, FIG. 5.

At 1404, an etch is performed into the back side of the semiconductorsubstrate to form a periodic structure on the back side of thesemiconductor substrate. See, for example, FIG. 6.

At 1406, a high absorption layer is formed lining the back side of thesemiconductor substrate. See, for example, FIG. 7. The high absorptionlayer has a low energy bandgap (e.g., less than about 1 eV and/or lessthan that of the non-porous semiconductor layer), such that the highabsorption layer advantageously has a high absorption coefficient. Thehigh absorption layer may be, for example, silicon germanium, ormonocrystalline silicon dope with a chalcogen.

At 1408, a passivation layer is formed covering the high absorptionlayer on the back side of the semiconductor substrate. See, for example,FIG. 8.

At 1410, in some embodiments, a carrier substrate is bonded to the backside of the semiconductor substrate through the passivation layer andthe high absorption layer. See, for example, FIG. 9.

At 1412, in some embodiments, the semiconductor substrate is thinnedthrough the front side of the semiconductor substrate. See, for example,FIG. 10.

At 1414, a pixel sensor is formed on the front side of the semiconductorsubstrate, where the pixel sensor comprises a photodetector in thesemiconductor substrate and the high absorption layer. See, for example,FIG. 11. Advantageously, by arranging the photodetector in the highabsorption layer, the photodetector has high quantum efficiency.

At 1416, an interconnect structure is formed covering the pixel sensoron the front side of the semiconductor substrate, where the interconnectstructure comprises wiring layers and vias alternatingly stacked withthe wiring layers. See, for example, FIG. 12.

At 1418, a color filter and a micro-lens are formed stacked over thepixel sensor on the front side of the semiconductor substrate. See, forexample, FIG. 13.

While the flowchart 1400 of FIG. 14 is illustrated and described hereinas a series of acts or events, it will be appreciated that theillustrated ordering of such acts or events is not to be interpreted ina limiting sense. For example, some acts may occur in different ordersand/or concurrently with other acts or events apart from thoseillustrated and/or described herein. Further, not all illustrated actsmay be required to implement one or more aspects or embodiments of thedescription herein, and one or more of the acts depicted herein may becarried out in one or more separate acts and/or phases.

With reference to FIGS. 15-23, a series of cross-sectional views1500-2300 of some embodiments of a method for manufacturing a BSI imagesensor with high absorption pixel sensors is provided. For example, themethod may be employed to manufacture the BSI image sensor of FIG. 4B.

As illustrated by the cross-sectional view 1500 of FIG. 15, a pluralityof high absorption pixel sensors 102 a-102 c are formed on a front-sidesurface 104 f of a semiconductor substrate 104. Further, in someembodiments, an isolation structure 214 is formed to demarcate regionsof the semiconductor substrate 104 corresponding to the high absorptionpixel sensors 102 a-102 c. The high absorption pixel sensors 102 a-102 cand/or the isolation structure 214 may, for example, be formed asdescribed with regard to FIG. 11.

As illustrated by the cross-sectional view 1600 of FIG. 16, aninterconnect structure 216 is formed over the high absorption pixelsensors 102 a-102 c and the semiconductor substrate 104. Theinterconnect structure 216 may, for example, be formed as described withregard to FIG. 12.

As illustrated by the cross-sectional view 1700 of FIG. 17, in someembodiments, the structure of FIG. 16 is flipped vertically and bondedto a carrier substrate 402. The bonding may, for example, be performedas described with regard to FIG. 9.

As illustrated by the cross-sectional view 1800 of FIG. 18, in someembodiments, the semiconductor substrate 104 is thinned. The thinningmay, for example, be performed as described with regard to FIG. 10.

As illustrated by the cross-sectional view 1900 of FIG. 19, in someembodiments, an etch is performed into the semiconductor substrate 104to divide the semiconductor substrate 104 into a non-poroussemiconductor layer 104 n and a porous semiconductor layer 104 p. Theporous semiconductor layer 104 p is along the back-side surface 104 b ofthe semiconductor substrate 104, and the non-porous semiconductor layer104 n is along the front-side surface of the semiconductor substrate104. The porous semiconductor layer 104 p may, for example, be formed asdescribed with regard to FIG. 5.

As illustrated by the cross-sectional view 2000 of FIG. 20, a periodicstructure 106 is formed in the back-side surface 104 b of thesemiconductor substrate 104. The periodic structure 106 may, forexample, be formed as described with regard to FIG. 6.

As illustrated by the cross-sectional view 2100 of FIG. 21, a highabsorption layer 110 is formed lining the back-side surface 104 b of thesemiconductor substrate 104. The high absorption layer 110 may, forexample, be formed as described with regard to FIG. 7.

As illustrated by the cross-sectional view 2200 of FIG. 22, apassivation layer 112 is formed over the high absorption layer 110. Thepassivation layer 112 may, for example, be formed as described withregard to FIG. 8.

As illustrated by the cross-sectional view 2300 of FIG. 23, a pluralityof color filters 404 are formed respectively over the high absorptionpixel sensors 102 a-102 c. Further, a plurality of micro-lenses 406 areformed respectively over the color filters 404. The color filters 404and/or the micro-lenses 406 may, for example, be formed as describedwith regard to FIG. 13.

With reference to FIG. 24, a flowchart 2400 of some embodiments of themethod of FIGS. 15-23 is provided.

At 2402, a pixel sensor is formed on a front side of a semiconductorsubstrate, where the pixel sensor comprises a photodetector in thesemiconductor substrate. See, for example, FIG. 15.

At 2404, an interconnect structure is formed covering the pixel sensoron the front side of the semiconductor substrate, where the interconnectstructure comprises wiring layers and vias alternatingly stacked withthe wiring layers. See, for example, FIG. 16.

At 2406, in some embodiments, a carrier substrate is bonded to the frontside of the semiconductor substrate through the interconnect structure.See, for example, FIG. 17

At 2408, in some embodiments, the semiconductor substrate is thinnedthrough a back side of the semiconductor substrate. See, for example,FIG. 18.

At 2410, in some embodiments, a first etch is performed into the backside of the semiconductor substrate to divide the semiconductorsubstrate into a porous semiconductor layer and a non-poroussemiconductor layer. See, for example, FIG. 19.

At 2412, a second etch is performed into a back side of thesemiconductor substrate to form a periodic structure along the back sideof the semiconductor substrate. See, for example, FIG. 20.

At 2414, a high absorption layer is formed lining the back side of thesemiconductor substrate. See, for example, FIG. 21.

At 2416, a passivation layer is formed covering the high absorptionlayer on the back side of the semiconductor substrate. See, for example,FIG. 22.

At 2418, a color filter and a micro-lens are formed stacked over thepixel on the front side of the semiconductor substrate. See, forexample, FIG. 23.

While the flowchart 2400 of FIG. 24 is illustrated and described hereinas a series of acts or events, it will be appreciated that theillustrated ordering of such acts or events is not to be interpreted ina limiting sense. For example, some acts may occur in different ordersand/or concurrently with other acts or events apart from thoseillustrated and/or described herein. Further, not all illustrated actsmay be required to implement one or more aspects or embodiments of thedescription herein, and one or more of the acts depicted herein may becarried out in one or more separate acts and/or phases.

In view of the foregoing, some embodiments of the present applicationprovide an image sensor. A semiconductor substrate includes a non-poroussemiconductor layer. The non-porous semiconductor layer is along a frontside of the semiconductor substrate. A periodic structure is along aback side of the semiconductor substrate. The periodic structureincludes a plurality of protrusions defined by the semiconductorsubstrate. A high absorption layer lines the periodic structure on theback side of the semiconductor substrate. The high absorption layer is asemiconductor material with an energy bandgap less than that of thenon-porous semiconductor layer. A photodetector is in the semiconductorsubstrate and the high absorption layer.

Further, other embodiments of the present application provide a methodfor manufacturing an image sensor. A semiconductor substrate isprovided. The semiconductor substrate includes a non-poroussemiconductor layer on a front side of the semiconductor substrate. Anetch is performed into a back side of the semiconductor substrate toform a plurality of surface protrusions on the back side. The surfaceprotrusions are formed in a periodic pattern, and the back side isopposite the front side. A high absorption layer is formed lining theplurality of surface protrusions on the back side of the semiconductorsubstrate. The high absorption layer is a semiconductor material with alower energy bandgap than that of the non-porous semiconductor layer. Aphotodetector is formed in the semiconductor substrate and the highabsorption layer.

Further yet, other embodiments of the present application provide animage sensor. A semiconductor substrate includes a non-poroussemiconductor layer and a porous semiconductor layer. The non-poroussemiconductor layer is along a front side of the semiconductorsubstrate. The porous semiconductor layer is along a back side of thesemiconductor substrate that is opposite the front side. A highabsorption layer lines the porous semiconductor layer on the back sideof the semiconductor substrate. The high absorption layer has a higherabsorption coefficient than the non-porous semiconductor layer. Aphotodetector includes a first doping-type region and a seconddoping-type region. The first doping-type region is in both thenon-porous semiconductor layer and the porous semiconductor layer. Thesecond doping-type region is in the high absorption layer and has anopposite doping type as the first doping-type region. The first andsecond doping-type regions interface to define a photo junction. Atransfer transistor is on the front side of the semiconductor substrate.The transfer transistor includes a source/drain region, and thesource/drain region is the first doping-type region.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method for manufacturing an image sensor, themethod comprising: providing a semiconductor substrate, wherein thesemiconductor substrate comprises a non-porous semiconductor layer on afront side of the semiconductor substrate; performing an etch into aback side of the semiconductor substrate to form a plurality of surfaceprotrusions on the back side, wherein the surface protrusions are formedin a periodic pattern, and wherein the back side is opposite the frontside; forming a high absorption layer lining the plurality of surfaceprotrusions on the back side of the semiconductor substrate, wherein thehigh absorption layer is a semiconductor material with a lower energybandgap than that of the non-porous semiconductor layer; forming aphotodetector in the semiconductor substrate and the high absorptionlayer, the photodetector comprising a first doping-type region and asecond doping-type region having opposite doping types and collectivelydefining a PN junction; and forming a pixel transistor on the front sideof the semiconductor substrate after forming the high absorption layer,wherein a source/drain region of the pixel transistor is the firstdoping-type region.
 2. The method according to claim 1, wherein thesemiconductor substrate further comprises a porous semiconductor layeron the back side of the semiconductor substrate, and wherein theproviding of the semiconductor substrate comprises: performing a secondetch into the non-porous semiconductor layer to form the poroussemiconductor layer from a portion of the non-porous semiconductorlayer.
 3. The method according to claim 2, wherein the etch is performedselectively into the porous semiconductor layer to form the plurality ofsurface protrusions.
 4. The method according to claim 1, wherein theforming of the photodetector comprises implanting dopants into the frontside of the semiconductor substrate to form the first doping-typeregion.
 5. The method according to claim 1, wherein the high absorptionlayer is formed of silicon germanium, or is formed of monocrystallinesilicon doped with a chalcogen.
 6. The method according to claim 1,further comprising: forming a passivation layer covering the highabsorption layer on the back side of the semiconductor substrate;bonding a carrier substrate to the back side of the semiconductorsubstrate through the passivation layer; and thinning the semiconductorsubstrate at the front side of the semiconductor substrate.
 7. Themethod according to claim 1, wherein a top surface of the highabsorption layer and a bottom surface of the high absorption layer havesaw-toothed profiles.
 8. The method according to claim 1, wherein theforming of the high absorption layer comprises implanting a chalcogeninto a portion of the semiconductor substrate to form the highabsorption layer from the portion of the semiconductor substrate.
 9. Amethod for forming an image sensor, the method comprising: patterning asemiconductor substrate to form a periodic structure in thesemiconductor substrate; after the patterning, forming a semiconductorlayer directly on and lining the periodic structure, wherein thesemiconductor layer has an energy bandgap different than an energybandgap of the semiconductor substrate, wherein the forming of thesemiconductor layer comprises implanting dopants into a portion of thesemiconductor substrate to form the semiconductor layer from the portionof the semiconductor substrate, and wherein the portion of thesemiconductor substrate decreases in energy bandgap from a beginning ofthe implanting to an end of the implanting; and forming a photodetectorin the semiconductor substrate and the semiconductor layer.
 10. Themethod according to claim 9, wherein the dopants comprise sulfur,selenium, tellurium, or any combination of the foregoing.
 11. The methodaccording to claim 9, wherein the forming of the semiconductor layercomprises supersaturating the semiconductor layer with a chalcogen. 12.The method according to claim 9, further comprising: implanting carboninto the semiconductor substrate to form a carbon-rich region lining theperiodic structure.
 13. The method according to claim 9, wherein themethod further comprises: performing an etch into a non-poroussemiconductor layer to form a porous semiconductor layer from a portionof the non-porous semiconductor layer, wherein a remainder of thenon-porous semiconductor layer and the porous semiconductor layer definethe semiconductor substrate, and wherein the periodic structure isformed in the porous semiconductor layer.
 14. The method according toclaim 9, wherein the semiconductor layer is formed with a top surfaceand a bottom surface that each conforms to the periodic structure andhas a periodic cross-sectional profile.
 15. The method according toclaim 9, wherein the photodetector comprises a first doping-type regionand a second doping-type region, wherein the first and seconddoping-type regions have opposite doping types and collectively define aPN junction, and wherein the method further comprises: forming a pixeltransistor on the semiconductor substrate, wherein the pixel transistoris formed before the patterning, and wherein a source/drain region ofthe pixel transistor comprises the first doping-type region.
 16. Amethod for forming an image sensor, the method comprising: forming anabsorption-enhancement structure on a first side of a semiconductorsubstrate, wherein the first side is more absorptive of radiation than asecond side of the semiconductor substrate, opposite the first side, dueto the absorption-enhancement structure; forming a semiconductor layeron the absorption-enhancement structure, wherein the semiconductor layerand the absorption-enhancement structure directly contact at aheterojunction; and forming a photodetector in the semiconductorsubstrate and the semiconductor layer from the second side of thesemiconductor substrate.
 17. The method according to claim 16, whereinthe forming of the absorption-enhancement structure comprises performingan etch into the first side of the semiconductor substrate to form asystematic structure of nanopores in the first side.
 18. The methodaccording to claim 16, wherein the forming of the absorption-enhancementstructure comprises patterning the first side of the semiconductorsubstrate with a periodic pattern.
 19. The method according to claim 16,further comprising: depositing a dielectric layer on the semiconductorlayer, wherein the semiconductor layer has a first surface facing thesemiconductor substrate, and further has a second surface facing thedielectric layer, and wherein the first and second surfaces are onopposite sides of the semiconductor layer and have a saw-toothedprofile.
 20. The method according to claim 16, wherein the forming ofthe semiconductor layer comprises depositing the semiconductor layer byepitaxy while doping the semiconductor layer with a chalcogen.